The growth characteristics and loss of viability of a marine bacterium, Vibrio alginolyticus 138-2 and its primary Na^+ pump defective mutant, Nap 1 were observed under several different experimental conditions.The difference between 138-2 and Nap1 became more prominent with decreasing organic content, implying that the primary Na^+ pump has more ecoogical significance under low nutrient condition, which is prevailing in most of marine environments.The growth of Nap 1 became more Na^+ concentration dependent at higher pH.Because V.alginolyticus uses Na^+ cycle at higher pH,Nap 1 has to face more energetic problem at alkaline condition.The same situation was also clearly shown in the case of survival at medium without any organic compounds.It is concluded that the primary Na^+ pump has a significant energetic role in Vibrio alginolyticus.In addition, the ecological significance of sodium driven polar flagellum was also investigated with special reference to its role on the bacterial attachment.It was clarified that attachment of V.alginolyticus to glass surface is dependent on sodium concentration or swimming speed.This finding is contrary to the general concept by DLVO theory, which state that bacterial motility is not enough to overcome the surface electrical repulsion.Further investigation has been undertaken to establish a new concept of bacterial attachment due to flagellar rotation.